Important new data out from Pew Research Center today. It turns out that the conventional wisdom about poll results was right.

Political operatives have long suspected that traditional phone polls skew in favor of Republicans, because landline users are more likely to fit GOP’s core demographics (older, more settled, more rural). Now the folks at Pew have done a retrospective analysis of 2010 midterm polls, comparing polls done with landlines only to those that included cell phones.

The results: landline-only polls skewed 5.1% in favor of GOP candidates. That is to say,

Across three Pew Research polls conducted in fall 2010 — conducted among 5,216 likely voters, including 1,712 interviewed on cell phones — the GOP held a lead that was on average 5.1 percentage points larger in the landline sample than in the combined landline and cell phone sample.

For progressive candidates who lost on Nov. 2, this is no comfort. But going forward, it provides a cautionary tale for pundits who hang on every poll result and build election narratives based on that data. Would the story of 2010 have been any different if the Republican’s perceived momentum wasn’t so inflated?

Read the full report here.